Flywheel energy storage

Flywheel energy storage (FES) works by spinning a rotor (flywheel) and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel. While some systems use low mass/high speed rotors, other use very massive rotors and correspondingly much lower rotational speeds, referred to as grid-scale flywheel energy storage. Other designs employ high-speed rotors that operate at tens of thousands of revolutions per minute.